1. Field of the Invention
The present invention relates to a laser range finder, and more particularly to a high precision laser range finder with an automatic peak control (APC) loop for increasing the measuring precision.
2. Description of Related Art
A laser range finder is one of the most important devices for measuring distances. Conventionally, the laser range finder employs a pulse type of a laser transmitter to transmit short laser pulse signals onto a target. A reflected laser signal from the target is received by a laser receiver to evaluate the distance, wherein the laser receiver must be a low noise, high sensitivity device. The distance between the target and the laser range finder is obtained by using the formula (I) shown below.
Td=2L/Cxe2x80x83xe2x80x83(I)
where L represents the distance, C represents the speed of light, and Td is a delay time between the firing laser pulse signal and the received laser pulse signal that is reflected from the target. A precise distance can be obtained by precisely measuring Td according to formula (I). In order to precisely measure the time delay Td, there are several functions of the laser range finder which need to be improved. These improvements are shown as following:
1. The transmitted laser pulse signal needs to be limited to as narrow as possible.
2. The received laser pulse signal from the target need to be stabilized, thus the fluctuation of timing start point in the received laser pulse signal caused by the different reflectivity of the target is kept at minimum.
3. A high sensitivity time delay measurement circuit is needed to improve the resolution of distance measurement.
In U.S. Pat. No. 3,921,095, a startable phase-locked loop technique is used to improve the delay time measurement resolution to picosecond order. In this patent, a startable oscillator is provided. The frequency of the startable oscillator can be locked to be a reference frequency whose initial phase at the instant of starting the startable oscillator is able to be maintained with a known precision. However, the startable oscillator is only workable in a high speed clock, especially in an ECL logic gate, thus the power consumption of this circuit is a main problem in a battery powered product.
In U.S. Pat. No. 5,075,878, a sampling circuit is used to improve the delay time measurement. In this patent, a more accurate delay time is derived through using a reference signal sampled in correlation to the fired laser pulse signal and the received laser pulse signal. The reference signal is used to derive a sampled waveform signal that has a period which is long relative to the fired and received laser pulse signal. However, in this patent, a distortion problem may occur in the sampling circuit, and consequently a measurement error may also occur in the sampling circuit.
To overcome these shortcomings, the present invention tends to provide a high precision laser range finder with an automatic peak control loop to mitigate and obviate the aforementioned problems.
The object of the present invention is to provide a high precision laser range finder with an automatic peak control loop for stabilizing the peak amplitude of a received laser pulse signal from a target without incurring influence by different reflectivity of the different targets so as to increase measurement precision.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the attached drawings.